1. Field of Invention
This invention relates to apparatuses for generating seismic waves and more particularly for generating seismic waves below the surface of the earth in a formation surrounding a borehole.
2. Description of the Prior Art
In exploring for oil, gas, and other mineral deposits in subsurface earth formations, seismic waves, (i.e., body waves or P- and S-waves), are used to map subsurface geologic structures and stratigraphic features. Seismic waves might also be useful for facilitating the production of oil and gas where such waves are used to vibrate an oil or gas bearing formation, thereby enhancing the movement of oil or gas in the formation (e.g., see U.S. Pat. No. 3,952,800).
Several typical types of seismic exploration techniques which use one or more boreholes include vertical seismic profiling (VSP), reverse vertical seismic profiling (RVSP), and crosswell seismic profiling. In the case of VSP, seismic energy sources such as explosive charges, surface vibrators, and other energy sources are placed on the surface of the earth and a plurality of seismic detector elements (geophones or hydrophones) are placed in a borehole. RVSP involves placing an array of geophones over the surface of the earth, while the energy sources such as explosives, vibrators, or pneumatic guns are placed in the borehole for generating an acoustic signal. Crosswell seismic profiling involves placing an array of geophones or hydrophones in one borehole and the energy sources in a neighboring borehole. In each of these techniques, the acoustic signals produced by the energy sources penetrate the formation surrounding the borehole(s) and are reflected by the subsurface strata to the geophones located either on the surface or in a neighboring borehole. Also, geophones or hydrophones can be placed in the well containing the source (i.e., the source well) to detect, for example, reflected energy from bodies, such as salt domes, adjacent to the borehole. Such imaging may be referred to as single well (reflection) profiling.
The reflected acoustic signals provide geologists and geophysicists acoustic images of the subsurface formations. Such acoustic images of the subsurface strata, and in particular of oil and gas reservoirs, can be constructed by a variety of techniques known as reflection seismic or tomographic imaging. Reflection seismic images the interfaces between different formations, while tomographic imaging provides information about formation velocities which can be used to help identify the presence of oil, gas, or mineral deposits.
A problem arises, though, where seismic information is needed over extended distances in the earth. Currently, no commercially available downhole seismic source has a horizontal range reliably exceeding 1500 feet. Typical wells in the most productive oil fields, however, are at least 3000 feet apart. Therefore, current downhole seismic source apparatuses are applicable to only a small percentage of oil fields with closely spaced wells.
Most downhole seismic sources operate unreliably at ranges over 1500 feet because the signal (comprised of P- and S-waves) to background noise ratio is low, which in turn provides poor quality acoustic images for characterizing the formation structure. The principal reason for obtaining a low signal to noise ratio is that most of the energy produced by a downhole acoustic source typically causes the propagation of tube waves in the liquid filled borehole, rather than producing the desired P- and S- waves.
Tube waves are propagated through a liquid contained in a borehole casing when a pressure pulse from an acoustic source is generated in the liquid. Most of the tube waves' energy propagates through the liquid as they move along a borehole. Tube waves are damped to a certain extent by liquid-solid frictional forces, but generally, the damping affect of such forces on tube waves is only slight. Tube waves, therefore, usually travel substantial distances through a liquid filled casing at a relatively constant velocity with little attenuation to their amplitude. Because of the existence of and efficient propagation of tube waves we can say that a borehole is an efficient acoustic waveguide. A downhole seismic source which is placed in such a waveguide directs most of its energy into creating guided waves (i.e., tube waves) rather than body waves. This is a major disadvantage in terms of loss of energy to tube waves and subsequent noise created at any receivers placed around the well containing the seismic source.
One type of apparatus which attempts to improve the signal to noise ratio by producing a standing pressure wave and thereby preclude energy losses in tube wave formation is disclosed by Kennedy et al. in U.S. Pat. No. 4,671,379. Kennedy et al.'s apparatus oscillates the borehole liquid in a selected portion of the borehole to establish a resonant standing pressure wave of desired frequency within the fluid. The standing wave is contained within the selected portion of the borehole, however, by using two movable air bladders whose spatial separation is varied from about 15 to about 100 feet. This device, therefore, suffers from several practical defects. First, the bladders have to move substantial distances (e.g., up to about 50 feet) as the source sweeps the desired frequency range (e.g., 20-100 Hz) over a 45 second time period. Consequently, the device does not operate reliably because downhole moving parts are exposed to borehole drilling fluids containing abrasive particles and debris. Second, the bladders have to be pressurized with an air hose extending downhole from the surface which is inefficient and inconvenient from an operational perspective. Third, because the bladders cannot move very quickly, the device's minimum frequency range sweep is about 45 seconds. It is well known that for land surface vibration sources better data is usually generated by using a succession of shorter sweeps in the range of three to ten seconds per sweep.
Another type of apparatus which attempts to improve the signal to noise ratio is a downhole device for converting tube waves into body waves. This tube wave converting device is disclosed by Winbow et al. in U.S. Pat. No. 4,993,001. The device is a constricting obstacle placed downhole. As tube waves are forced to squeeze by the constriction, additional acoustic pressure is created on the borehole wall which leads to the formation of body waves. A disadvantage of this scheme, however, is that very high energy tube waves are required, which in turn leads to forming strong background noises as the tube waves enter the borehole and when they impact the end of the borehole.
Another downhole device related to that described in U.S. Pat. No. 4,993,001 is a broadband resonant wave downhole seismic source disclosed by Winbow et al. in U.S. Pat. No. 5,268,537. The device is used to partially or completely block off the borehole and create a fluid-filled borehole cavity. The fluid inside the cavity is oscillated to establish a standing pressure wave which is radiated through the wellbore into the surrounding formation. However, this device functions most effectively at high frequencies (i.e., greater than about 1,500 Hz). It is well known that lower frequencies (less than about 1000 Hz) are preferable for routine reflection seismic and tomographic imaging work.
Accordingly, there is a need for an efficient and reliable apparatus for producing an acoustic signal in a borehole having an increased ratio of P- and S-waves to background noises and thereby an enhanced signal to noise ratio.